1. Field of the Invention
The present invention generally relates to a position location system and, more particularly, to a method and apparatus for mitigating multipath effects at a satellite signal receiver.
2. Description of the Related Art
Global Positioning System (GPS) receivers use measurements from several satellites to compute position. GPS receivers normally determine their position by computing time delays between transmission and reception of signals transmitted from satellites and received by the receiver on or near the surface of the earth. The time delays multiplied by the speed of light provide the distance from the receiver to each of the satellites that are in view of the receiver. The GPS satellites transmit to the receivers satellite-positioning data, so called “ephemeris” data. In addition to the ephemeris data, the satellites transmit to the receiver absolute time information associated with the satellite signal, i.e., the absolute time signal is sent as a second of the week signal. This absolute time signal allows the receiver to unambiguously determine a time tag for when each received signal was transmitted by each satellite. By knowing the exact time of transmission of each of the signals, the receiver uses the ephemeris data to calculate where each satellite was when it transmitted a signal. Finally, the receiver combines the knowledge of satellite positions with the computed distances to the satellites to compute the receiver position.
More specifically, GPS receivers receive GPS signals transmitted from orbiting GPS satellites containing unique pseudo-random noise (PN) codes. The GPS receivers determine the time delays between transmission and reception of the signals by comparing time shifts between the received PN code signal sequence and internally generated PN signal sequences.
Each transmitted GPS signal is a direct sequence spread spectrum signal. The signals available for commercial use are provided by the Standard Positioning Service. These signals utilize a direct sequence spreading signal with a 1.023 MHz spread rate on a carrier at 1575.42 MHz (the L1 frequency). Each satellite transmits a unique PN code (known as the C/A code) that identifies the particular satellite, and allows signals transmitted simultaneously from several satellites to be received simultaneously by a receiver with very little interference of any one signal by another. The PN code sequence length is 1023 chips, corresponding to a 1 millisecond time period. One cycle of 1023 chips is called a PN frame.
Each received GPS signal is constructed from the 1.023 MHz repetitive PN pattern of 1023 chips. At very low signal levels, the PN pattern may still be observed, to provide unambiguous time delay measurements, by processing, and essentially averaging, many PN frames. These measured time delays are called “sub-millisecond pseudoranges”, since they are known modulo the 1 millisecond PN frame boundaries. By resolving the integer number of milliseconds associated with each delay to each satellite, then one has true, unambiguous, pseudoranges. The process of resolving the unambiguous pseudoranges is known as “integer millisecond ambiguity resolution”. A set of four pseudoranges together with the knowledge of the absolute times of transmissions of the GPS signals and satellite positions at those absolute times is sufficient to solve for the position of the GPS receiver. The absolute times of transmission are needed in order to determine the positions of the satellites at the times of transmission and hence to determine the position of the GPS receiver.
The process of measuring pseudoranges in a GPS receiver begins with a procedure to search for the GPS signals in the presence of noise by attempting a series of correlations of the incoming signal against the PN codes. The search process can be lengthy, as both the exact frequency of the signal and the time-of-arrival delay are unknown. To find the signal, GPS receivers traditionally conduct a two dimensional search, checking each delay possibility at every possible frequency. To test for the presence of a signal at a particular frequency and delay, the GPS receiver is tuned to the frequency, and the incoming signal is correlated with the known PN code delayed by an amount corresponding to the time of arrival. If no signal is detected, the search continues to the next delay possibility, and after all delay possibilities are checked, continues to the next frequency possibility. Each individual correlation is performed over one or more milliseconds in order to allow sufficient signal averaging to distinguish the signal from the noise. A correlation between the incoming signal and the PN code results in a “peak” at the code delay position where the PN code of the incoming signal matches the known PN code.
Conventionally, GPS receivers use few correlators per channel to track the GPS PN codes, such as one early-late (E-L) correlator pair per channel. E-L correlator spacing is usually from ±0.5 PN code chips to ±0.05 code chips. Blockage of the direct satellite signal will cause the true correlation peak to fade, while the presence of a reflected signal will cause a false peak to appear offset from the true peak. If the true correlation peak fades, and a reflected signal causes a false peak to appear more than 0.5 chips away from the (now invisible) true peak, then this reflected signal is invisible to the E-L correlator pair and it causes no measurement error. However, upon initially searching for a GPS signal (as opposed to tracking a GPS signal), E-L correlator pairs are susceptible to large errors if only the reflected signals are visible. Moreover, E-L correlator pairs do not provide for high sensitivity in low signal-to-noise ratio environments.
To increase sensitivity at the GPS receiver, designers add additional correlators per channel. For a given sensitivity level, the addition of correlators decreases search times in proportion to the number of additional correlators added to the channel. In contrast to an E-L correlator pair, however, a GPS receiver with a large number of correlators per channel will “see” a false peak from a reflected signal, even if the reflected signal suddenly appears more than one chip away from the true peak. Since the false peak is visible, the GPS receiver may produce erroneous measurements. When one or more reflected signals are received by the receiver, the signals form “multipath” components that can result in one or more false peaks or peak distortion. There are known techniques for mitigating multipath effects when the reflected signal overlaps a direct signal, causing a distortion in the correlation peak. These techniques, usually based on narrow correlator spacing, somewhat mitigate the distortion effect of multipath components in the presence of a direct-path signal. However, in some instances of receiver use, the entire direct signal path may be blocked resulting in the receiver only receiving the reflected signals. In those instances, the existing techniques are ineffective at mitigating reception errors do to multipath components.
Accordingly, there exists a need in the art for an improved method and apparatus for mitigating multipath effects at a satellite signal receiver.